During the past 4 years, we have conducted a pilot study to determine the effect of inhaled nitric oxide on oxygenation in infants with severe, ventilator dependent BPD. The only modification that has occurred in the study protocol in the past year is a change in the principal investigator from Roberta A Ballard to Beverly A Banks. There have been no other protocol modifications. Infants are eligible for enrollment if they are over 4 weeks old, and ventilator dependent with a mean airway pressure > 10 cm H2O and an oxygen requirement of > 45%. Infants are excluded from the study if they have congenital heart disease. All infants enrolled are treated with inhaled nitric oxide at 20 ppm for a 3 day period. During that time, inspired oxygen concentration is adjusted to maintain saturations greater than 92%. At the end of 3 days, infants are evaluated to determine their response to nitric oxide therapy. Infants are considered to have a beneficial response if they are able to wean by 10% or more on their inspired oxygen concentration. If an infant has no beneficial response, nitric oxide is weaned every 6 hours as tolerated until it can be discontinued. Infants who are felt to have had a beneficial response to the initial 3 day course are maintained on inhaled nitric oxide for a prolonged course of therapy. Every three days an attempt is made to wean the nitric oxide by 20%. If the oxygen saturation falls by more than 10% during the wean attempt, then the nitric oxide is increased to the previously tolerated level, and weaning is attempted again after 3 days. Thirty-two preterm infants, age 4 weeks to 7 months, (23-32 weeks gestational age), have been enrolled to date, with two infants receiving 2 separate courses of iNO. We are currently conducting a randomized, double blind, crossover design study of iNO in more moderate BPD